1. Field of the Invention
This invention relates to a polycrystalline diamond body having improved utility and improved properties or characteristics including significantly improved adhesion properties for attachment to metal or other materials used for tool supports or holders. Polycrystalline diamond is now commonly used in a composite compact form. This composite compact is comprised of a polycrystalline diamond portion intimately bonded to a cemented tungsten carbide backing. The carbide backing of this composite compact is mounted onto a tool support or holder for subsequent drilling, turning, cutting or similar operations which take advantage of the super wear resistant diamond surface. Polycrystalline diamond is also used without the carbide backing and after the chemical catalyst/binder needed for polycrystalline growth has been leached out in an acid bath. The leached polycrystalline is then set into a metal matrix for subsequent use. This invention also relates to the method of making the improved polycrystalline diamond body during an ultra high pressure and temperature press cycle, as well as the method of making the improved polycrystalline diamond body with post press techniques.
Throughout the following disclosure it should be understood that the term "diamond" is intended to cover all super hard abrasive materials including but not limited to synthetic or natural diamond, cubic boron nitride and wurtzite boron nitride as well as combinations thereof.
2. Prior Art
Diamond with its unsurpassed wear resistance is the most effective material for many industrial applications such as machining and earth drilling. Because of the high costs involved in mining natural diamond or manufacturing synthetic diamond, its relatively brittle characteristics, and its size limitation, it is impractical to fabricate entire tools from diamond. Consequently, diamond is used with a tool holder or support made from metal or other appropriate materials. The utilization of diamond in tools has, however, been limited because diamond is not readily attachable to metal or other materials. This non-attachability is most likely due to the inertness of diamond and the difference in the chemical structure of the surface of diamond and that of other materials.
An early solution to this problem of non-attachability involved setting single crystal diamond into a metal matrix by surrounding the single crystal diamond with molten metal and allowing the metal to form around the diamond. In particular, more than half of a single crystal diamond body is surrounded and held in a pocket of metal such that the diamond is mechanically locked in the metal. The exposed portion of the diamond is sharpened to a point and held against a workpiece for drilling, turning or cutting or other related purposes. This system, however, is costly as it requires a substantial quantity of diamond which is unexposed and useless for cutting or drilling purposes. Another problem with this system is that it uses single crystal diamond. In particular, single crystal diamond tends to fracture when a large or sudden force is applied in the direction of one of its planes of cleavage. Consequently, when single crystal diamond is fixedly set into a metal matrix, limitations are imposed upon the angles at which it can be used. This problem with the planes of cleavage is not limited to applications where single crystal diamond is set into a metal matrix. Rather, in substantially all industrial applications single crystal diamond will tend to fracture if subjected to forces in the direction of the plane of cleavage.
Diamond in its polycrystalline form has an added toughness over single crystal diamond due to the random distribution of the crystals so that there are not particular planes of cleavage. Therefore polycrystalline diamond is frequently the preferred form of diamond in many drilling, turning, cutting or similar operations and has been directly substituted for single crystal diamond for use in a metal matrix.
Polycrystalline diamond can be manufactured in a press in which grains of diamond and other starting materials are subjected to ultrahigh pressure and temperature conditions. The methods of making poylcrystalline diamond in a press are well known in the art and further detailed description thereof is not considered necessary. Polycrystalline diamond, however, like single crystal diamond is relatively non-wettable and its surface does not readily attach or adhere to other materials. Two of the most common techniques which have been used with polycrystalline diamond to circumvent this problem of non-attachability are first, forming a composite compact which is polycrystalline diamond with a cemented tungsten carbide backing and brazing this composite compact to a holder or support and second, setting polycrystalline diamond without a carbide backing into a metal matrix.
In the brazing application, it is first necessary to "grow" polycrystalline diamond directly onto a carbide substrate by placing a cemented carbide piece and diamond grains mixed with a catalyst binder into a container of a press and subjecting it to a press cycle using ultrahigh pressure and temperature conditions. As a result, at the ultrahigh temperature and pressure needed for polycrystalline formation, the resulting polycrystalline diamond body is intimately bonded to the carbide piece resulting in a composite compact being in the form of a layer of polycrystalline diamond intimately bonded to a carbide substrate. The polycrystalline diamond body in the form of a composite compact is then attachable to other materials through the exposed surface of the carbide backing by conventional soldering or brazing methods employing relatively low temperatures at which the polycrystalline diamond structure remains stable. The use of polycrystalline diamond in the composite compact form has become the standard for industrial application of polycrystalline diamond where it is brazed to other materials.
One problem, however, with composite compacts arises during the cooling of the composite compact after it has been formed which stresses the polycrystalline diamond structure. The diamond and carbide bond is formed in the press at a temperature in the range of 1300.degree.-2000.degree. C. At this temperature, the compact is stable. As the compact cools, however, a residual stress arises at the diamond carbide interface due to a difference in the coefficients of thermal expansion of the two materials. In particular, the carbide substrate has a higher coefficient of thermal expansion than the polycrystalline diamond body so that it contracts more than the polycrystalline diamond body during the cooling period. This difference can cause the polycrystalline diamond structure to fracture either during the cooling process resulting in rejections or after cooling and during use of the polycrystalline diamond.
A second problem with composite compacts arises with the use of cobalt or other metal catalyst/binder systems which are often used for polycrystalline growth. After crystalline growth is complete, the catalyst/binder remains within the pores of the polycrystalline structure. Because cobalt or other metal catalyst/binders have a higher coefficient of thermal expansion than diamond, when the composite compact is heated, e.g., during the brazing process by which it is attached to a holder or during actual use, the metal from the binder expands at a higher rate than the diamond. Therefore, when the compact is subjected to temperatures above a critical level, fractures throughout the polycrystalline diamond structure result. Another disadvantage of leaving a catalyst/binder within the pores results from the fact that the catalyst used to lower the energy of activation required for polycrystalline growth during the press cycle will likewise lower the energy of activation required for polycrystalline degradation. In other words, the presence of a catalyst in the pores causes polycrystalline degradation to occur at a lower temperature than when the catalyst is not present in the pores thus placing another limitation on the temperature to which the compact can be subjected.
A third problem which often arises from the composite compact involves the migration of the cobalt binder present in the most commonly used type of cemented carbide phase into the pores of the diamond layer. In particular, cobalt or other metal binders are always present in cemented carbide. When a carbide substrate and diamond grains are subjected to the ultrahigh pressures and temperatures needed for intercrystalline diamond growth, the cobalt or other metal binder present in the carbide migrates into the polycrystalline diamond phase. This migration into the polycrystalline diamond phase has no substantial effects on the polycrystalline diamond structure when the binder used in the polycrystalline diamond phase is the same as that in the cemented carbide substrate. However, when a binder system which is different from that present in the cemented carbide substrate is used in the polycrystalline diamond phase for polycrystalline growth, e.g., silicon, the migration of the cobalt or other material from the substrate into the polycrystalline diamond phase results in undesirable contamination of the polycrysalline diamond portion. This alters the properties of the polycrystalline diamond and may limit its utility. For example, by reducing the thermal stability of the polycrystalline diamond. Therefore when a cemented carbide substrate is used, the choice of catalyst/binder systems is limited.
A second common use of polycrystalline diamond involves setting polycrystalline diamond without a carbide backing into a metal matrix. In particular, the polycrystalline diamond is placed in a mold into which metal powder is added. Subsequent heating cements the metal powder around the diamond. For this use, the polycrystalline diamond is grown in a press after which it is placed in an acid bath. The acid bath leaches out the catalyst/binder which remains in the pores of the polycrystalline diamond. The acid also dissolves any cemented carbide backing, which of course, precludes using an acid treated polycrystalline body in the brazing application. The removal of the binder from the pores of the polycrystalline diamond results in a more stable structure which has a higher degradation temperature so that it withstands the temperature required to cement the metal powder of the matrix.
There are, however, a number of problems associated with the use of polycrystalline diamond set into a metal matrix. First, as with single crystal diamond, a substantial quantity of diamond which is unexposed and useless for cutting or drilling purposes is necessary. Second, the number of cutting surfaces is restricted as a substantial portion of the diamond body is surrounded by the metal matrix. Third, because of the relatively deep pocket of matrix needed to hold the polycrystalline diamond body in the tool, failure often occurs when the matrix material wears away and the diamond body accordingly becomes dislodged.